The Erlang-C calculations are described step by step below,
using and example of 360 calls per half hour, with an average call duration of
4 minutes, and 55 agents. The target answer time for service level is 15
seconds.

The first parameter needed is the average customer arrival rate.
It doesn't matter what time unit is used to specify the arrival rate, as long
as the same time unit is used for the average call duration. Also, the results
we shall get for waiting time will be in these time units.

The term "traffic intensity" comes from the original application
of Erlang-C, which was for telephone networks, and the volume of calls was
described as the "traffic". We need to calculate the traffic intensity as a
preliminary step to the rest of the calculations.

(5) Calculate agent occupancy

The agent occupancy, or utilisation, is now calculated by
dividing the traffic intensity by the number of agents. The agent occupancy
will be between 0 and 1. If it is not less than 1 then the agents are
overloaded, and the Erlang-C calculations are not meaningful, and may give negative
waiting times.

(6) Calculate the Erlang-C formula

Now we can calculate the main Erlang-C formula. This formula
looks complicated, but is straightforward to calculate with a few lines of
programming. The value of EC(m,u) is needed to calculate the answers
we actually want.

(7) Calculate probability of waiting

EC(m,u) is the probability that a call is not answered immediately,
and has to wait. This is a probability between 0 and 1, and to express it as a
percentage of calls we multiply by 100%.

(8) Calculate average speed of answer (ASA)

Having calculated EC(m,u) it is quite easy to calculate
the average waiting time for a call, which is often referred to as the
"Average Speed of Answer" or ASA. We have to remember the time units we used for
arrival rate and call duration.

(9) Calculate service level

Frequently we want to calculate the probability that a call will
be answered in less than a target waiting time. The formula for this is given
here. Remember that, again, the probability will be on the scale 0 to 1 and
should be multiplied by 100 to express it as a percentage.

(10) Calculate agents needed

If the service level is specified and you want to calculate the
number of agents needed, then you must do a bit of (intelligent) trial and error.
You have to find the number of agents that will just achieve the service-level
you want. A good starting point is the traffic intensity, rounded up to the next integer.
the increase the number of agents until the required service-level is reached.